1. Field of the Invention
The present invention relates in general to a unit cell for use in a solid polymer electrolyte fuel cell that employs a solid polymer electrolyte membrane, and to a solid polymer electrolyte fuel cell wherein a plurality of unit cells are stacked.
2. Description of the Related Art
Fuel cells have been noticed as to their ability to be an electric generator. As one example of these fuel cells, a solid polymer electrolyte fuel cell is known generally. As is well known, the solid polymer electrolyte fuel cell is able to produce electrical power by means of an electrochemical reaction when supplied with oxygen (air) as an oxidant and hydrogen as a fuel, these being supplied onto the surfaces of a pair of catalyst electrodes superposed against either face of an electrolyte which is a solid polymer electrolyte membrane, such as a solid ion exchange membrane or the like.
In solid polymer electrolyte fuel cells, it is important that these be consistent supply of oxygen and hydrogen onto the surfaces of the catalyst electrodes in order to consistently and efficient produce the intended voltage. It is also important for the appropriate temperature to be maintained. Accordingly, there is typically employed a cell of a structure wherein a membrane/electrode assembly (MEA) composed of a breathable porous membrane oxidant electrode and a fuel electrode disposed on either side of the solid polymer electrolyte membrane is assembled with a first separator superposed against the oxidant electrode face thereof and a second separator superposed against the fuel electrode face thereof. A plurality of such unit cells are stacked and electrically connected directly to produce the desired voltage.
An oxidant gas flow passage is formed by means of covering with the oxidant electrode a recess disposed on the first separator, and fuel gas flow passage is formed by means of covering with the fuel electrode a recess disposed on the second separator. A coolant flow passage is formed by a recess disposed in a secondary face of the first separator or second separator on the back side from a primary face which is superposed against the electrode, by covering the recess with the secondary face of another adjacent cell.
At respective peripheral edges of stacked unit cells, there are formed perforating therethrough in the stacking direction an oxidant gas inlet and an oxidant gas outlet, a fuel gas inlet and a fuel gas outlet, and a coolant inlet and a coolant outlet. The oxidant gas inlet and oxidant gas outlet are connected to both ends of the oxidant gas flow passage, whereby oxidant gas is supplied to the membrane/electrode assembly through the oxidant gas flow passage. Likewise, the fuel gas inlet and the fuel gas outlet are connected to both ends of the fuel gas flow passage, thereby supplying fuel as to the membrane/electrode assembly through the fuel gas flow passage. In addition, the coolant inlet and coolant outlet are connected to both ends of the coolant flow passage, circulating coolant through the coolant flow passage.
However, these solid polymer electrolyte fuel cells of conventional construction may still suffer from disability in producing sufficient electrical power. Namely, for ensuring an excellent capability of electric power generation, it is important that the membrane/electrode assembly and separators be assembled together with a high precision alignment. Since the membrane/electrode assembly employed in the conventional fuel cell is considerably thin, it was significantly difficult to dispose the membrane/electrode assembly between the first and second separators with a high precision alignment thereto.
JP-A-11-045729 or other documents propose a use of a frame disposed covering the peripheral edge of the solid polymer electrolyte membrane of the membrane/electrode assembly. With this arrangement, the membrane/electrode assembly is reinforced, making it possible to precisely install the membrane/electrode assembly between the first and second separators with ease and precision.
In order to produce an electric voltage with stability, the solid polymer electrolyte fuel cell shall should ensure an stable supply of the oxygen gas (oxygen) and the fuel gas (hydrogen) to respective surfaces of the catalyst electrodes. For the stable supply of these gases, it is important to prevent a leakage of gases through gaps formed between the separators or between the separator and the membrane/electrode assembly. To meet this ends, the conventional solid polymer electrolyte fuel cell, as shown in JP-A-2000-294254 etc., sealing rubber layers are disposed between the adjacent ones of the mutually stacked membrane/electrode assemblies and separators so that the membrane/electrode assemblies and separators are stacked on one another via the sealing rubber layers, thereby preventing gas leakage through gaps between the stacked members. Typically, the sealing rubber layers are arranged for surrounding the fuel gas flow passage and the oxidant gas flow passage.
However, the sealing rubber layer employed in the conventional solid polymer electrolyte fuel cell may cause the gas leakage problem at around connecting portions between the gas flow passages and gas inlets/outlets. Described more specifically, the sealing rubber layer is arranged for fringing the gas flow passages and the gas inlets/outlets connected to the gas flow passages. Therefore, if the membrane/electrode assembly of a flat plate shape is smaller than the separators and located inside the gas inlets/outlets, the peripheral edge portion of the membrane/electrode assembly is partially located on the gas flow passage and the connecting flow passage, leading to a risk that gas fed to one side of the membrane/electrode assembly leak into the other side. That is, since the solid polymer electrolyte membrane is a sheet considerably thin and readily deformable, its peripheral edge portion placed on and exposed to the flow passages will be readily deformed due to gas feeding pressure, thereby producing gaps between the separators and the membrane/electrode assembly. Thus, there was a risk that the fuel gas and the oxidant gas could leak around the peripheral edge of the membrane/electrode assembly into the other side.
Another measure has been proposed to prevent the gas leak around the peripheral edge of the membrane/electrode assembly. For instance, it may be possible to form a connecting flow passage by utilizing a groove open in the primary face of the separator. The opening of the groove is covered by means of a rigid lid member fitted thereon in a bridging fashion, so that the connecting flow passage is of tunnel construction. This lid member is provided with a sealing rubber layer bonded thereto. By sandwiching the peripheral edge portion of the membrane/electrode assembly (solid polymer electrolyte fuel cell) between the sealing rubber layer formed on the separator and the sealing rubber layer formed on the lid member, the gas leak around the peripheral edge of the membrane/electrode assembly can be prevented. This measure is able to prevent the gas leak effectively, but on the other hand, it needs an additional component independent of the separator, resulting in the increased number of components and deterioration in production efficiency due to the need for a precise alignment of the lid member.